Your user can log into cpanel and click change language button and select a new language. (the button is defined as a folder with A B C on it in the x themes)

Or:

Log into ssh. Change directory to /var/cpanel/users . grep -l 'LANG=Choose' `ls` . this will produce a list of files which need to be edited. You can then pico the file and change LANG=Choose a Language to LANG=english or similar.

If anyone wants to post a sed, awk, grep solution im sure people would appreciate it.